Fault severity assessment for rolling element bearings using the Lempel–Ziv complexity and continuous wavelet transform

This paper proposes a new version of the Lempel–Ziv complexity as a bearing fault (single point) severity measure based on the continuous wavelet transform (CWT) results, and attempts to address the issues present in the current version of the Lempel–Ziv complexity measure. To establish the relationship between the Lempel–Ziv complexity and bearing fault severity, an analytical model for a single-point defective bearing is adopted and the factors contributing to the complexity value are explained. To avoid the ambiguity between fault and noise, the Lempel–Ziv complexity is jointly applied with the CWT. The CWT is used to identify the best scale where the fault resides and eliminate the interferences of noise and irrelevant signal components as much as possible. Then, the Lempel–Ziv complexity values are calculated for both the envelope and high-frequency carrier signal obtained from wavelet coefficients at the best scale level. As the noise and other un-related signal components have been largely removed, the Lempel–Ziv complexity value will be mostly contributed by the bearing system and hence can be reliably used as a bearing fault measure. The applications to the bearing inner- and outer-race fault signals have demonstrated that the revised Lempel–Ziv complexity can effectively measure the severity of both inner- and outer-race faults. Since the complexity values are not dependent on the magnitude of the measured signal, the proposed method is less sensitive to the data sets measured under different data acquisition conditions. In addition, as the normalized complexity values are bounded between zero and one, it is convenient to observe the fault growing trend by examining the Lempel–Ziv complexity.